I. Field of the Invention:
This invention relates to a gamma correction circuit for the picture signal used in the TV signal transmission end.
II. Description of the Prior Art:
As is known in this field, the output of a TV camera has to be gamma corrected because of the characteristics of a cathode-ray (picture) tube in the receiver end. The reason will be explained more in detail.
The luminance (brightness) signal voltage is not linearly proportional to the luminance of the screen in the typical picture tube. When the luminance signal voltage is low, luminance variation is small respect to with a unit change in voltage. However, when the luminance signal voltage is high, luminance variation is great with respect to the same, which represents an exponential curve. Namely, the light output from the picture tube is not proportional to the voltage driving it. When a picture signal including a luminance signal proportional to the luminance of the picture image taken by a TV camera is broadcasted from the transmission end without any correction, the luminance of the picture image displayed at the receiver end differs from its actual luminance. In order to avoid this inconvenience, it is required that the luminance signal obtained by the TV camera is corrected to have the characteristics opposite to the exponential characteristics and thereafter is broadcasted from the transmission end.
A circuit for this purpose is generally called a gamma correction circuit which can correct the characteristics of the luminance signal at the transmission end so as to make them coincide with the characteristics of the picture tube at the receiver end. FIGS. 1, 3 and 5 show conventional gamma correction circuits, respectively; FIGS. 2A and 2B and FIGS. 4A and 4B respectively show input and output signals therefrom.
Referring to FIG. 1, the gamma (.gamma.) correction circuit comprises an amplifier 11 which includes an input transistor Q1, a transistor Q2 to which base a DC bias voltage V1 is applied, resistors R1 and R2, and DC current sources I1 and I2. It further comprises a variable impedance circuit 12 onnected to the amplifier 11 including a transistor Q3, a resistor R3, a diode D1 and a DC current source I3, and a variable impedance circuit 13 connected also to the amplifier 11 including a transistor Q4, a resistor R4, a diode D2 and a DC current source I4. The amplifier 11 amplifies an input video signal supplied to an input terminal IN, which is constituted by a luminance signal and a chrominance signal, using a gain of the ratio of the resistor R2 to the resistor R1. When an output from the amplifier 11 exceeds a DC base bias voltage V2 of the transistor Q3 in the variable impedance circuit 12, the input video signal is amplified by a gain determined by a ratio of a parallel resistance R2-3 of the resistors R2 and R3 to the resistor R1. Furthermore, when the output from the amplifier 11 exceeds a DC base bias voltage V3 of the transistor Q4, the input video signal is amplified by a gain determined by a ratio of a parallel resistance R2-3-4 of the resistors R2, R3 and R4 to the resistance of the resistor R1. It should be noted that the summed resistance of parallel-connected resistors e.g. R2 and R3 will be referred to "a parallel resistance R2-3" in the specification.
The operation of this conventional circuit is to vary gains stepwise. Specifically, a change in gain is substantially approximated by a polygonal approximation method to obtain finally desired gamma characteristics. When a triangular wave which only includes the luminance signal shown in FIG. 2A is supplied to the input terminal IN as the input video signal, a luminance signal which has been gamma corrected is obtained as shown in FIG. 2B.
In the conventional circuit of the type described above, if the gains of the amplifier and the variable impedance circuits, i.e., the operating points of the respective variable impedance circuits are properly set, a predetermined output level can be obtained independently of values used for gamma correction. However, in order to obtain desired values used for gamma correction, the bias voltages V2 and V3 of the variable impedance circuits 12 and 13 must be accurately determined. Since the values used for gamma correction greatly vary in accordance with the preset voltage level, the biasing voltages are difficult to be controlled. Moreover because this conventional circuit introduces the polygonal approximation method, there is essential defect for high precise .gamma.-correction.
FIG. 3 shows another type of the conventional gamma (.gamma.) correction circuit. This circuit is constituted by a buffer amplifier 21 and a variable impedance circuit 22 which comprises transistors Q11 and Q12, a resistor R12, diodes D11 and D12, and a DC current source I11. The output of the buffer amplifier 21 is connected via the resistor R11 to the variable impedance circuit 22. The input video signal is amplified by the buffer amplifier 21 and an outut video signal which has been gamma connected is produced at the node N1 between the resistr R11 and the variable impedance circuit 22.
In this prior art circuit, after the level of the signal appearing at the node N1 becomes higher than that of a DC bias voltage V0 for the transistor Q11, if the output level of the amplifier 21 is changed, then a current flowing through the resistor R11 and the diodes D11 and D12 is changed. As a result, since impedances of the diodes D11 and D12 are changed, the gamma correction is effected.
In the circuit described above, impedance change of the diodes D11 and D12 represents exponential curve, so that substantially ideal gamma characteristics can be realized. However, when a triangular wave signal which includes only the luminance signal as shown in FIG. 4A is supplied to an input terminal IN as the input video signal, the peak value of the gamma corrected signal delivered to an output terminal OUT varies greatly in accordance with the value used for gamma correction, as shown in FIG. 4B.
FIG. 5 shows still another type of the conventional gamma (.gamma.) correction circuit. The operation of this gamma correction circuit is simply described as follows. A gamma corrected signal by diodes D21 and D22 is produced from an emitter of a transistor Q21. A signal which is not gamma corrected is produced from an emitter of a transistor Q22. These two signals are mixed at the various mixture rate using a potentiometer VR so as to obtain a video output signal which has been properly gamma corrected.
However, in this circuit, the level of the signal obtained at the emitter of the transistor Q22 and the level of the signal obtained at the emitter of the transistor Q21 must coincide each other so as to obtain a composite gamma corrected output signal. Therefore, the level of the resultant output signal becomes very low. Furthermore, a large current must flow through resistors R21 and R22 of the base bias circuit of the transistors Q21 and Q22, resulting in large current consumption.